Along with the greatly increased use of computers for offices and for manufacturing facilities, there has developed a need for a cable which may be used to connect peripheral equipment to mainframe computers and to connect two or more computers into a common network. A number of factors must be considered in order to arrive at a cable design which is readily marketable for such uses.
Cable connectorability is very important and is more readily accomplished with twisted insulated conductor pairs than with any other medium. A widely used connector for insulated conductors is one which is referred to as a split beam connector. See, for example, U.S. Pat. No. 3,798,587 which issued on Mar. 19, 1974 in the names of B. C. Ellis, Jr. et al. Desirably, the outer diameter of insulated conductors of the sought-after cable is sufficiently small so that the conductors can be terminated with such existing connector systems.
The jacket of the sought-after cable should exhibit low friction to enhance the pulling of the cable into ducts or over supports. Also, the cable should be strong, flexible and crush-resistant, and it should be conveniently packaged and not unduly weighty. Because the cable may be used in occupied building spaces, flame retardance also is important.
To satisfy present, as well as future needs, the sought-after cable should be capable of suitable high frequency data transmission. This requires a tractable loss for the distance to be covered, and crosstalk and electromagnetic interference (EMI) performance that will permit substantially error-free transmission. Also, the cable must not contaminate the environment with electromagnetic interference.
The sought-after data transmission cable should be low in cost. It must be capable of being economically installed and be efficient in terms of space required. Generally, for cables in buildings, which are used for such interconnection, installation costs outweigh the cable material costs. Building cables should have a relatively small cross-section inasmuch as small cables not only enhance installation but are easier to conceal, require less space in ducts and wiring closets and reduce the size of associated connector hardware. At the same time, however, peripheral connection arrangements must meet attenuation and crosstalk requirements.
Another cost consideration is whether or not the system is arranged to provide transmission in what is called a balanced mode. In balanced mode transmission, voltages and currents on the conductors of a pair are equal in amplitude but opposite in polarity. This requires the use of additional components, such as transformers, for example, at end points of the cable between the cable and logic devices thereby increasing the cost of the system. Generally, computer equipment manufacturers have preferred the use of systems characterized by an unbalanced mode because most of the industry is not amenable to investing in additional components for each line. In an unbalanced mode transmission system, voltages and currents on the conductors of a pair are not characterized by equality of amplitude and opposition of polarity. However, given other advantages of a balanced system such as, for example, less crosstalk particularly at longer distances, computer equipment manufacturers may be inclined to install such a system.
Of importance to the design of local area network copper conductor cables are the speed and the distances over which data signals must be transmitted. In the past, this need has been one for interconnections operating at data speeds up to 20 kilobits per second and over a distance not exceeding about 150 feet. This need has been satisfied in the prior art with single jacket cables which may comprise a plurality of insulated conductors that are connected directly between a computer, for example, and receiving means such as peripheral equipment. Additional components at the ends of each pair to convert to the balanced mode have not been used.
In today's world, however, it becomes necessary to transmit data signals at much higher speeds over distances which may include several thousands of feet. Both the data rates and the distances for transmission may be affected significantly by the topology of some presently used local area network arrangements. In one, for example, each of a plurality of terminal stations is connected to a common bus configured in a ring such that signals generated at one station and destined for another must be routed into the wiring closet and seriatim out to each station intermediate the sending and receiving stations. The common bus, of course, requires a very high data rate to serve a multiplicity of stations and the ring configuration doubles the path length over which the data signals must be transmitted from each station to the wiring closet.
Even at these greatly increased distances, the transmission must be substantially error-free and at relatively high rates. Often, this need has been filled with coaxial cable comprising the well-known center solid and outer tubular conductor separated by a dielectric material. The use of coaxial cables, which inherently provide unbalanced transmission, presents several problems. Coaxial connectors are expensive and difficult to install and connect, and, unless they are well designed, installed and maintained, can be the cause of electromagnetic interference. Of course, the use of coaxial cables does not require components such as transformers at each end to provide balanced mode transmission, but the size and connectorization of coaxial cables outweigh this advantage.
Shielding often is added to a twisted pair of insulated conductors to confine its electric and magnetic fields. In this way, susceptibility to electromagnetic interference is reduced. However, as the electric and magnetic fields are confined, resistance, capacitance and inductance all change, each in such a way as to increase transmission loss. One company markets a cable in which each pair of conductors is provided with a shield and a braid is provided about the plurality of pairs. In order to compensate for the increased losses, the conductor insulation must be increased in thickness. As a result, the insulated conductors cannot be terminated with conventional connector hardware.
On the other hand, a cable shield surrounding all conductor pairs in a cable may be advantageous. Consider that the pairs may be inside a cabinet and may be exposed a high speed digital signals. Stray radiation will be picked up in the longitudinal mode of the twisted pairs. If the pairs are then routed outside the cabinet, they may radiate excessively. If there is a cable shield enclosing the plurality of pairs, the shield may be grounded at the cabinet wall so that the shield will not itself carry stray signals to the outside environment. Thus, a shield disposed about all the pairs in a cable can be effective in preventing electromagnetic interference and yet not increase appreciably the attenuation of each pair.
The sought after cable should be one that may be used to replace the well known D-inside wiring which comprises a plurality of twisted insulated conductor pairs. The pairs are non-shielded and are enclosed in a jacket. Improved pair isolation has long been sought in such wiring to reduce crosstalk. Hopefully, the cable of this invention also could be used for burglar alarm systems and for today's sophisticated thermostat systems, for example.
Seemingly, the solutions of the prior art to the problem of providing a local area network cable which can be used to transmit, for example, data bits error-free at relatively high rates over relatively long distances have not yet been totally satisfying. What is needed and what is not provided by the prior art is a cable which is compatible with balanced or unbalanced mode transmission equipment and which can be readily installed, fits easily into building architectures, and is safe and durable.